The present invention relates to the use of a catalyst comprising a magnesium oxide/copper oxide/aluminum oxide spinel or a copper-impregnated copper oxide/aluminum oxide spinel for catalytic reduction of nitrogen oxides (NO.sub.x) in exhaust gases of internal combustion engines and/or combustion power plants.
Published German Patent Application No. DE 43 01 470 discloses a CuAl.sub.2 O.sub.4 spinel to which tin, lead and an oxide, a salt or an elemental form of an element of the second main or subsidiary group of the Periodic Table has been added, and subsequently calcined to form the final spinel. The spinel is used to decompose N.sub.2 O. However, this patent does not disclose the use of this catalyst for catalytic oxidation of CO and/or the catalytic reduction of NO.sub.x, particularly at temperatures of several hundred degrees Centigrade.
A spinel is a composition of the general chemical formula A.sub.a B.sub.b O.sub.4, which has, at least microscopically, a crystallographic or crystalline cubic lattice structure with face-centered oxygen ions and with tetrahedral or octahedral gaps. The A elements and up to 50% of the B elements are disposed in the tetrahedral gaps and the remaining B elements are disposed in the octahedral gaps. The term A element or B element refers here to the crystallographic arrangement. From a material point of view, the A and B elements can be different from one another.
For environmental reasons, the purification of gases, particularly of exhaust gases which are produced by internal combustion engines, for example, especially, by diesel engines and engines operating on a lean mixture, as well as the decrease in the CO content of exhaust gases and the denitrification of these gases, that is, the decomposition of nitrogen oxides (NO.sub.x), is a pressing problem.
Published European Patent Application No. EP 042 471 discloses a catalyst which is used for the catalytic oxidation of carbon monoxide (CO) to carbon dioxide (CO.sub.2) and contains the metals copper (Cu), zinc (Zn) and aluminum (Al) as metal oxides. At least 60% of the Cu present is bound to the aluminum oxide as a copper oxide/aluminum oxide spinel. In the free spaces of the porous spinel, zinc oxide is disposed in an amount of 1% to 20% by weight.
The catalyst of EP 042 471 has a purifying action only with respect to the CO, which is based on the catalytic oxidation of carbon monoxide. However, EP 042 471 does not disclose whether the catalyst is suitable for the purification of NO.sub.x and/or hydrocarbons produced by internal combustion engines or combustion power plants.
It is an object of the present invention to develop a catalyst that has good stability in the presence of exhaust or waste gas components and good purification action, particularly with respect to NO.sub.x.
Pursuant to the present invention, these and other objects are accomplished by a catalyst comprising a magnesium oxide/copper oxide/aluminum oxide spinel having the chemical formula: EQU Mg.sub.A Cu.sub.B Al.sub.C O.sub.4
wherein A+B+C.ltoreq.3 and A&gt;0, B&gt;0 and C&gt;0.
Furthermore, these and other objects are also accomplished by a catalyst comprising a copper-impregnated copper oxide/aluminum oxide spinel having the chemical formula: EQU Cu.sub.A Cu.sub.B Al.sub.2 O.sub.4
wherein A+B=1 and A&gt;0, and B&gt;0.
By using a Mg.sub.A Cu.sub.B Al.sub.C O.sub.4 or a Cu.sub.A Cu.sub.B Al.sub.C O.sub.4 spinel, produced by a method disclosed in Published German Patent Application No. DE 43 01 470, the catalyst is largely stable with respect to exhaust gas components such as H.sub.2 O, NO.sub.x, CO.sub.2 and/or SO.sub.2 and acts in an oxidizing manner on hydrocarbons and carbon monoxide and in a catalytically reducing manner on NO.sub.x, particularly on NO and NO.sub.2. The reduction of NO.sub.x takes place in an oxygen-containing gas in the presence of reducing agents such as hydrocarbons. Advantageously, the hydrocarbons are present in sufficient concentration in the exhaust gas of internal combustion engines, particularly of diesel engines. Moreover, it has proven to be advantageous to provide these spinels with catalytically active elements such as palladium, platinum, rhodium, ruthenium, osmium, iridium, rhenium and/or rare earth metals such as lanthanum and cerium, vanadium, titanium, niobium, molybdenum, tungsten and/or salts thereof and/or oxides thereof.
According to the present invention, substoichiometric compounds in which Al.sub.2 O.sub.3 functions as a matrix and which have characteristic spinel lines in the X-ray spectrum, are also regarded as spinels. The spinel of the composition AAl.sub.2 O.sub.4 is present in the Al.sub.2 O.sub.3 matrix, so that a stoichiometry of A.sub.(1-x) Al.sub.2 O.sub.4 results.
In a preferred embodiment, the catalyst comprises a copper oxide/magnesium oxide/aluminum oxide spinel of the chemical formula EQU Mg.sub.(1-B) Cu.sub.B Al.sub.2 O.sub.4
wherein 0&lt;B&lt;1.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.